The present invention relates to a pneumatic radial tire more particularly an improved tread portion capable of improving high speed durability, high speed handling, ride comfort, tire weight and the like.
In general, pneumatic tires used under high speed conditions such as radial tires for sports cars, passenger cars and the like are provided in the tread portion with tread reinforcing belts. The belts include a breaker wound on the radially outside of a carcass and a band wound outside the breaker. Here, the band has a very small cord angle with respect to the tire equator, for example, 0 plus minus 5 degrees, and the breaker has a relatively large cord angle. As shown in FIG. 7, such a band belt include one or two full-width bands C2 and optionally one or two edge bands C1. The edge bands C1 are disposed in only the axial edge portions for the purpose of preventing belt edge separation caused by a centrifugal force during high speed running. The full-width bands C1 are disposed as a radially outermost belt ply for high speed handling performance.
Such a belt structure greatly increases the rigidity of the tread portion, which as show in FIG. 5, hinders the inside tread half (a) from contacting with the ground during cornering, and as a result, as show in FIG. 6, the ground contacting area changes its shape from a broader rectangular shape (b) to a undesirable narrow triangular shape (b1). Even in wide-tread tires whose aspect ratio is less than 60%, it is difficult to maintain a preferable ground contacting shape. Therefore, road grip during cornering decreases, and transitional cornering characteristics are changed greatly. Thus, it is difficult to improve high speed cornering performance, high speed maneuverability and the like. Further, it is difficult to improve ride comfort and to decrease the belt weight or the tire weight.